Traffic control system



Dec. 23, 1952 M. WALLACE TRAFFIC CONTROL SYSTEM 4 Sheets-Sheet 1 Filed June 16, 194:7

1 I I I I I I I I I I I I -H MWIIM.

mmvrok, MARCEL WALLAC E I I I I I I I I I I I I I I I I I I I I I I ATTORNEY Dec. 23, 1952 M. WALLACE 2,523,208

TRAFFIC CONTROL SYSTEM Filed June 16, 1947 4 Sheets-Sheet 2 INVENTOR.

MA RCEL WALLACE ATTORNEY -m mm Dec. 23, 1952 M. WAL-.LACE

TRAFFIC coNTRor. SYSTEM 4 Sheets-Sheet 3 Filed June 16, 1947 ATTORNEY Filed June 16, 1947 Dec, 23, 1952 M. WALLACE 4 Sheets-Sheet 4 FIE4 INVENTOR. MA RCEL WALLACE ATTORNEY Patented Dec. 23, 1952 TRAFFIC CONTROL SYSTEM Marcel Wallace, East Port Chester, Conn., as-

signor, by mesne assignments, of one-half to lsaid Wallace, doing business as Panoramic Laboratories, East Port Chester, Conn.

Application June 16, 1947, Serial No. 754,942

16 Claims.

This invention relates generally to air trac control systems, and more particularly to vsystems for providing at a central location continuous indications of navigational parameters associated with each of a plurality7 of aircraft. l

Briefly described, the invention concerns itself with the utilizationv of f omnidirectional beacon signals in conjunction with translating equipment aboard one or more aircraft, the translating equipment serving to transmute theV beacon signals into signalsrepresentative of the altitude, range and bearing of each of the aircraft, and for transmitting the latter signals to a ground station for indication there of such altitude, range and bearing.

The present system involves fundamentally a transmission, from each of a plurality of aircraft, of a signal having a frequency determined in accordance with the altitude of the aircraft. Such `signals may be interpreted at ground stations, in

terms of altitude,` as by means of suitably calibrated frequency 2scanning; or panoramic receivers. i' 1 The factor which determines the total frequency band requiredqfor transmission of a given range of altitudes, in systems of the above character, is primarilyy the frequency stability of the transmitters and of the lreceiverutilized inthe system, since frequency Vdrifts either of transmitted frequencies or of receiver tuning are indistinguishable fromfrequencychanges due to altitude variations of aircraft.k In my previous systems-as exemplified in myapplication for U. S. Patent Serial Number 729,378, February 18, 1947, now Patent Number;2,565, 008. I utilize independent oscillators at both receiver and transmitter, which are susceptible of drift for various reasons. Inthe systemof the present application, on the other-.handJ yutilize a novel principle, to wit, I providea common carrier or frequency whichfmay be utilized by the various transmitters as well as by thereceiver of the system as a frequency standard. .lTheIcommoncarrier is transmitted from the ground station and is received aboard each of kthe aircraft, and yretransmitted after having addedtheretof. by lheterodyn action a component-of frequency v.which depends upon altitude. The added component of .frequency is derived from a relatively lowfrequency oscillator, which is subjected tov frequency v.control in .accordance with aircraft altitude, and Abecause of its low frequencydriftingpf thei oscillator is of negligible magnitudewhen considered in'relation to the frequency value of the altitude representa- 2 is of the superheterodyne type, the common carrier is utilized as al source of local oscillation so that the difference between (l) signals received and representative of altitude and (2) the locally supplied. oscillation, depends only upon the frequency of the altitude representative lo'w frequency oscillator, the total effect of frequency drifts being cancelled as between the receiver and the various transmitters except for such slight drifts as may occur in the altitude controlled oscillators. The latter, as has been pointed out hereinbefore, are of slight relative magnitude in the overall system.

While the system as so far described does not depend upon the specific source of common carrier which may be utilized, it is a feature of the invention to utilize transmissions derived from omni-directional beacons for such purpose, since such beacons will be in any event available at all air stations, in accordance with present regulations of the Civil Aeronautics Authority. But such utilization .provides still a further advantage, which is not apparent at first blush, and appreciation of which requires consideration of the components and operation of the present standard' omni-directional beacon system.y

` U'I-h'e' omni-directionalrbeacon systemKWhich is presently standard equipment under the regulations of the Civilr Aeronautics Authority is well understood, and includes apparatus for transmitting an omni-directional pattern of radiation and a rotating pattern, the omni-directional pattern being amplitude modulated with a sub-carrier which is itself frequency modulated at a low frequency F. The rate ofY rotation of the rotating pattern corresponds with this low frequency F. Aboard each aircraft, therefore, are available two signals F, one of which has the same phase at every azimuthalV bearing, but the other of which has a phase which is a direct function of azimuthal bearing, since it derives from the rotation of the rotating pattern.' The bearings of aircraft may then bedetermined aboard such aircraft by a simple measurement of relative phase.

. We now recall that thehaltitude representative transmissions from each of the aircraft are com- `posited from received omni-directional beacon Y quency value may be indicated or recorded on a panoramic or frequency-scanning receiver.

The repeated o r transponded signals, representative of altitude, contain the very signals which are inherent' lthe beacon transmissions, and

which are utilized aboard the aircraft for bearing determination by phase comparison. These same signals may therefore be interpreted at the ground station in terms of the bearing of the signal repeating aircraft, by a process of phase comparison precisely analogous to that utilized aboard the aircraft.

In a-ccordance with the description so far, we have seen that the omni-directional beacon signals provided by the ground station may be utilized aboard aircraft for bearing determinations, may serve as a common carrier for stabilizing or standardizing the frequencies of the overall system with which we are concerned, by utilization thereof as a major component of altitude representative signals repeated from the aircraft, and the repeated signals contain modulations which are susceptible of interpretation on the ground in terms of bearing of the aircraft.

Still a further possibility is envisaged, based upon the following known laws of electricity. When a signal is transmitted from one point and repeated back from a remote location to that point the relative phase of the signals as transmitted and received back at the one point is representative of signal transmission time. The same effect is found in respect to modulations of signal, and systems are known, accordingly, in which a modulated high frequency carrier is transmitted from a ground station to an aircraft and thence repeated to the ground station, where the phases of the modulation as transmitted and received are -compared for purposes of deermination of range of the aircraft. By proper selection of the frequency of modulation various ranges may be efficiently measured, it being required generally, to avoid ambiguity, that the period of the modulation be greater than the maximum anticipated transmission time of the signals, s that phase comparison is effected between signals which have not changed phase by more than a single cycle in the course of transmission.

When the beacon transmissions are examined, in the present system, and as has been recited hereinbefore, a 10,000 cycle per second modulation is found present. This frequency readily lends itself to range determinations. Therefore, the very signals which, When received at the ground station after being repeated from the aircraft, enable determination of altitude and bearing of the aircraft, still further enable determination of range thereof by a simple phase comparison between the 10,000 cycle signal as transmitted by the beacon, and as received by the ground station after transponding via the aircraft.

It will be evident from the above discussion that the present system provides a maximum of navigational information for a minimum of equipment additionally to that which is in any event available and required to comply with regulations of the Civil Aeronautics Authority, and that the airborne equipment in particular is extremely simple, being represented merely by an altitude controlled transponder, and that the transponder requires no attention from the pilot in the performance of its functions.

At the ground station, of course, a bank of frequency and phase measuring equipments is required, so that transmissions from each of the aircraft may be analyzed and the resulting navigational information presented for visual inspection, a separate presentation being preferably provided for each aircraft. I realize that various types of indicating and/or recording systems may be utilized at the ground station, but I prefer to utilize a separate receiver at the ground station for each aircraft, each receiver being supplied with a phase comparator for range and a phase rcomparator for bearing, the comparators being utilized to control meters, which are calibrated in terms of range and bearing. The receivers may be of the frequency following types and may be locked to the altitude representative transmissions of selected aircraft. Altitudes, on the other hand, may be represented on a facsimile type recorder of the frequency scanning type, so that the altitudes of all craft may be commonly displayed on a single record receiving surface for ready comparison.

In operation, then, the altitudes of all aircraft adjacent to a beacon station, or an air station, may be continuously recorded on a time fed tape. A traffic control officer may scan the tape periodically and thereby keep accurate check on the altitudes of all aircraft. Should the traffic control officer desire azimuth and range information with respect to any specic aircraft, he has only to tune one of the indicating receivers to the altitude frequency o-f that aircraft, and read the indicating meters to derive the desired information. Once a receiver has been tuned to the altitude frequency of a selected aircraft, it remains tuned to that aircraft despite changes in altitude thereof, and thereby automatically provides a. continuous indication of the navigational parameters of that aircraft so long asthe traffic control officer may desire.

It should be realized that the system as above briefly described does not require, specifically, a beacon station, nor a 'beacon station of the type now approved by the Civil Aeronautics Authority. Nor need all phases of the system be utilized simultaneously. So, if one is interested in alti'- tude determinations only, a reference carrier may be supplied from the ground independently of any beacon systems, for use in accordance with my invention. Should it be desired to determine bea-ring, the ground carrier may be modulated with bearing representative signals of any desired type known to the beaconry art, so only that they are susceptible of being transponded and of being interpreted in terms of bearing after being transponded. Likewise, if range determinations are desired any steady modulation of suitable frequency may be imposed on the altitude representative carrier, and range may be determined by a simple phase lmeasurement as between the modulation as transmitted and the modulation as received at the ground station after being transponded.

I have disclosed my invention in terms of one specic mode of application thereof, to wit, as applied in connection with the Civil Aeronautics Authority approved beacon system, not because the invention requires that system, but merely as one practical, and for many purposes preferred, mode of application of the invention.

I realize that the CAA system may, with advantage, be modified, for correlation with my invention, as by selection of a more advantageous frequency for range determination than is afforded by the present 10,000 cycle per second signal, or by imposition on the beacon carrier of a special unmodulated signal, say at 2500 or 3000 cycles per second, the sole function of which may be to enable range determinations.

I consider the above suggestions and modifications to arise naturally and without -the exercise of the inventive faculty, from mybasic invention as herein disclosed.

In ac-cordance with the above it is a primary object of my invention to provide an improved navigational and telemetric system for aircraft.

It is a further object of the present invention to provide a telemetric system for` transmitting altitude representative frequency from each of a plurality of aircraft and for receiving and translating such signals at a central location, wherein the receiving and translating equipment as Well as the various airborne transmitting equipment, are stabilized from a single signal common thereto.

It is still a further object of the present invention to provide a system for transmitting a signal froma ground .station which has a phase relative to a reference signal whichv depends upon the azimuthal bearing of a receiving station relative to the ground station and for transponding the signal from the receiving station to the ground station to enable determination of the said azimuthal bearing.

It is another object of the present invention to provide a system for providing a carrier at an altitude representative frequency for transponding the azimuthal bearing representative signal, referred to in the previous paragraph.

It is still another object of the 'invention to provide a system of altitude and range determination for aircraft wherein. a signalis transmitted from a predetermined geographical location to one or more aircraft, and wherein each aircraft transponds the said signal to the predetermined location in terms of a modulation on a carrier at an altitude representative frequency, range being determinable by comparison of the phase of the signal as received after being transponded, and as transmitted.

It is still a further object of the invention to provide a system for transponding from an aircraft modulations derived from beacon signals, for determination at a remote point of the .characteristics of the beacon signals as transponded and for interpretation of those characteristics in terms of range and bearing of the transponding aircraft.

Still a further object of the invention `resides in the provision of a navigational system -which utilizes a common carrier as a primary frequency stabilizing element, and wherein further, the frequency of the common carrier is modified in accordance with the value of a telemetric quantity.

Another object of the invention resides in the provision of a telemetric system wherein a common carrier is supplied to frequency stabilize the elements of the system, including a plurality of telemetric transmitters and a common re,- ceiver therefor, and wherein the frequencies of the transmitters are caused to departfrom the frequency of the common carrier in accordance with the value of a telemetric quantity.

Still another object of the present invention resides in the provision of a telemetric system wherein a common carrier is supplied to frequency stabilize the elements of thesystem, that common carrier being modulated with signals which when received at a remote location have acquired distinguishing characteristics depending upon the location of the remote location with respect to the point of origin of the common carrier, and wherein the common carrier is frequency translated at the remote location in accordance with the value of a telemetric quantity substantially Without disturbance to the disd tinguishing characteristics, and is then retransmitted to a signal analyzing station for analysis and indication of the value of the telemetric quantity and of the distinguishing characteristics.

The above and still further objects, advantages and features of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment of the invention, especially when taken in conjunction with the appended drawings, wherein:

Figure 1 is a functional block diagram of a telemetric transponder constructed in accordance with the present invention;

Figure 2 is a functional block diagram of an omni-directional beacon system as presently approved by the Civil Aeronautics Authority;

Figure 3 is a functional block diagram of a central receiving, recordingand indicating station for translating signals provided by the tele-l metric transmitters of Figure 1;

- Figure 4 is a functional block diagram of a: phase comparison system for bearing as utilized'.

in the present invention; and

Figure 5 is a functional block diagram of a'.

phase comparison system for range in accord-V ance with the present invention. Y

Referring now more `particularly ,to the draw-l ings, I describe an embodiment of an airborne equipment in accordance with the present in vention. p

Referring particularly to Figure ,1 there is: shown an antenna I which supplies signals tol a radio frequency amplifier 2, Since signals received by antenna I derive Vfrom a ground beacon station the antenna I and the amplifier 2 are designed to be relatively broad band, and capable of handling any signal which may bek encountered in a given omni-'directional beacon system. At the present time such signals may be found in the range -125 mc., and I have selected, for the purpose of example, a representative frequency of mc., as the input signal fs, to the amplifier 2, the system operating identically, however, for any other desired frequency.

The output of amplifier 2 is applied to a mixer 3, to which is also applied the output of an oscillator 4, which provides a frequency fo, which in my present example equals 110 mc. The difference frequency between fs and fo is derived from the mixer A3 by means of I. F. amplifier 5, operating at a frequency of fs-fo or 10 mc., and the output of amplifier 5 may be detected in a detector 6, the detected audio output amplified in an audio amplifier '1, and the output of the latter applied to a reproducer 8, such as a speaker or phones. Y

The elements I-8 inclusive together comprise, therefore, a conventional superheterodyne type receiver R which is available for receiving signals over the phone channel which'forms part of every omni-directional beacon, and as will appear hereinafter. n Y y Y i w Y The output of the mixer 3 vmay be further applied to a bearing analyzer S, whichfsupplies an output current Ydependent on theV relative phases of two modulation signals Fc and Fp,4v derivable from the omni-directional beacon.

signals, the meter lil serving to measure theoutput current, and hence being calibratable in terms of azimuthal bearing.

The output of the I. amplifier 5 may "be:k farther Supplied. t0. enger.. It@..whi91i-iealeg supplied a signal deriving from a tunable oscillator I2, 'the output frequency of which is controlled in accordance with an altitude measurement provided by an aneroid cell I3, in known manner. The frequency fc of the altitude controlled oscillator I2 may be selected of relatively low value, when compared to ,fs or fo, and may be caused to vary over a relatively slight range in response to the aneroid cell control unit I3. For purposes of exemplication only, I have suggested a value of fc=67 mc., the value 6 mc. correspon-ding with zero altitude, or altitude at sea level under standard conditions of atmospheric pressure, and the value 'I mc. corresponding with an altitude of 10,000 ft. under the same standard conditions. An amplifier I4 is coupled to the output of the mixer I I, w-hich serves to derive from the mixer a signal at frequency fs-fo-I-C, or the sum of the frequency present in the I. F. amplifier 5 and that generated by the oscillator I2. and that signal represents in respect to its frequency the altitude measured by the aneroid cell I3.

The output of the amplifier I4 is applied in turn to a mixer I5, to which is also applied signal deriving from oscillator 14, so that the output of mixer I5, as selected by tuned lamplifier I6 equals f+fsf0+fc=fs+fc It will be obvious then that the effect of the frequency fo on the output of the mixer I has been completely eliminated, and that the amplifier I6 carries a signal which is a composite of the beacon frequency fs and the altitude representative frequency fc, and may vary between the limits 126-127 mc. in accordance with the exemplary frequency values which I have adopted for purposes of discussion.

The frequency fs-I-Jc may be applied to a power amplifier I1 for transmission over an antenna I8.

The power amplifier I1 may have associated therewith a modulator I9, which m-ay be supplied with speech currents deriving from a microphone 20, over a filter 2I, which serves to pass only speech frequencies in the band G-4000 cycles, to -avoid interference with other functions of the equipment, to be described hereinafter. The elements I6-2I inclusive comprehend a relatively conventional speech transmitter T. There is accordingly required only the elements II-I5, for addition to a beacon receiver for receiving voice communication and bearing, and to a standard communication transmitter, in order to accomplish transmissions at frequencies controlled in accordance with altitude, and to so stabilize those frequencies that a relatively narrow range of frequencies serves for transmission of a wide range of altitudes, frequency drifts of the altitude representative signals being rendered negligible in the overall picture, by virtue of the stabilized value of the frequency fs and of the relatively small frequency selected for fc. Relatively great drifts of fc, accordingly, impose but a small percentage change in fs-I-fc.

Further, since all aircraft utilizing the present system and flying in the vicinity of the same beacon, utilize the identical value of fs, any drifts of ,fs are common to the entire system and do not introduce errors in relative altitude as between any plurality of aircraft of the system.

The modulator I9 may be supplied with coded transmission interrupting voltages, from a code keyer 22, which serves to interrupt transmissions from power amplifier I1 at relatively infrequent intervals, for relatively short time periods, in coded rhythm, a different code being assigned to all aircraft utilizing the system, and the coding providing a mode of identification of the transmitting aircraft.

Reference is now made to Figure 2 wherein is illustrated in functional rblock diagram the standard C. A. A. approved V. H. F. omni-directional beacon, a clear understanding of which is requisite to further understanding of the present invention. The exciter 50 represents a transmitter, which operates at the frequency fs, referred to in connection with the description of the embodiments of the invention illustrated in Figure 1 of the accompanying drawings, and which, for the sake of providing actual values, has been assumed to equal 120 mc. The carrier provided by the exciter 50 i-s applied to a modulator 5I, and thence to an omni-directional antenna 52. The modulator 5I is supplied with energy, over a lead 53, from a source of 10 kc. energy 54 (Fsc) which is in turn modul-ated in frequency at a 60 cycle rate (Fc) from a source of 60 cycle energy 55 The carrier radiated by the antenna 52, therefore, at frequency fs, is modulated so as to make available to suitable receivers a 60 cycle reference signal (Fc).

The output of the exciter 50 is further supplied to a pair of side frequency .generators 56 and 51, respectively. The generator 56 when supplied with modulation frequency, say at 60 cycles per second, provides side bands only, formed by the coaction of the carrier fs and the modulation. The generator 51 also provides side band energy only. Modulation is supplied to the side frequency generators 56 and 51 from the source 55, over a phase splitter 58, which provides two 60 cycle per second modulations differing in phase by one of which is supplied to the generator 56 and the other to the generator 51. The generators 56 and 51, respectively, feed a pair of antenna arrays. crossed at right angles to each other, one array comprising the pair of antennas 59, and the other array the pair of antennas 60.

By virtue of the angular space relationship between the arrays 59 and 60, and further by Virtue of the 90 phase disparity between the modulations of the carriers supplied to the arrays, respectively, a rotating field of radiant energy is provided, which rotates at the rate of 60 revolutions per second, and which when combined with the omni-directiona1 radiation of antenna 52, provides a rotating cardioid configuration of radiant energy.

Any receiver inthe eld of the antennas 59, 60 and 52 is therefore supplied with a carrier having a 10,000 cycle per second sub-carrier (Fsc) impressed thereon, which is in turn modulated by a 60 cycle per second signal (Fc), and the latter when finally abstracted by suitable demodulation processes from the carrier, has a fixed phase for all azimuthai positions of the receiver. The receiver also receives a signal (Fp) due to the rotating cardioid which, at the receiver, varies in intensity at a 60 cycle per second rate, due to the rotation of the cardioid pattern relative to the receiver, the phase of the signal when received being a function of the azimuthal position of the receiver. By comparison, then, of the phase of ythe reference signal (Fc) derived from the 10,000 cycle carrier, with that of the 6.0 cycle signal (Fp) of variable phase, a measure of azimuth may be provided. The required detection, analysis and phase comparison is accomplished by the bearing analyzer 9 and bearing is indicated aboard each aircraft by a suitably calibrated meter I 0 (Figure l), in a manner which need not concern us at this point.

The carrier supplied by the antenna 52 may be further modulated by impressing on amplitude modulator l voice signals from a microphone 6l, the voice signals having no components at 30 cycles per second or less, nor adjacent to 10,000 cycles per second, so that possible interference with the beaconing functions of the equipment will not occur.

The voice signals referred to are detected by detector 6 (Figure 1) and reproduced in reproducer 8 (Figure l).

The details of the beacon system forms no part of the present invention, and such beacons are Iin actual use at the present time, and have been Widely described in the literature. So far as concerns Letters Patent of the United States, closely analogous systems are found in Byrne #2,252,699 and Byrne #2,313,048. It therefore appears superfluous to extend the discussion of the omnidirectional beacon, per se. It will, however, be realized that the signal fs, as received by antenna i of Figure 1, will contain the modulations required for determination of azimuth, and that, therefore, the transmitted frequency Fs and fo (Figure 1) will also contain the same modulations.

The signals fs-l-fc, which is at a basic frequency representative of aircrafts altitude, is also modulated with signals representative of -aircrafts azimuthal bearing relative to an omni-directional beacon, since the signal fs-l-Jc contains the carrier fs, as received and complete with its modulations. It is, therefore, possible to provide a receiver at a ground station which may be tuned to the frequency fs-l-fc, the value of the tuningr providing a measure of altitude of the craft providing the signal fs-l-fc, and to provide in connection with the receiver a bearing analyzer, or a modulation analyzer, similar to 9, I0 (Figure 1), to provide readings of the azimuthal bearing :of the transponding aircraft.

The above does not exhaust the possibilities of Ithe present invention. I have identified -the reference modulation at 60 cycles per second, which is omni-directionally transmitted, by the symbol Fc. The 10,000 sub-carrier upon which is imposed Fc has been designated Fsc. The signal utilized for azimuthal rotation of the radiated pattern, at 60 C. P. S. has been designated Fp. It will then be realized that the signal Fsc has a phase which may be arbitrarily designated 0 when transmitted, and a phase @-I-q when re- -ceived at antenna I (Figure 1), and that, assuming negligible time consumedin traversing the Isystem of Figure 1 from antenna! to antenna I2,

'the signal Fsc will be received atthe ground stament of phase, to a distance of 10 miles, without any modification of the primary beacon system. One obstacle to 4such determination might, at first blush, seem insuperable, and that is the fact that the frequency Fsc is in fact frequencymodulated and hence presents no definite phase overa period Hof* time.v l ljhis obstacleisin fact easily overcome bynltering the signal Fsc through Aa Verynarrow altitudes.

10 band pass filter, which removes all side-bands therefrom, leaving a pure 10,000 cycle tone available for phase comparison.

Reference is now made to Figure 3 of the drawings, for illustration of an altitude indicating and recording system. An antenna, denoted by the reference numeral 80, is provided for receiving from aircraft signals which are representative of The signals intercepted by the antenna 30 are amplified in an .amplifier 8 I, having a band pass adequate for the entire altitude representative spectrum, and thence applied to a mixer 82, to which is also applied the frequency fs, which may be derived directly from the exciter 5s (Figure 3), over a cable 83. I have therefore applied to the mixer 82 a series of frequencies ,fs-l-fc, deriving from a plurality of aircraft, and a frequency fs. To the output of the mixer S2 is coupled an I. F. amplifier 8B, capable of passing the band of frequencies represen-ted by fc, i. e. 1 mc. Wide, inthe present example, extending from 6-7 mc.

It will be clear that since the frequency fs was added to the altitude varying frequency fc at the transmitters of the system and is now subtracted from the transmitted signals at frequency fs-l-c, that the effect of frequency drifts cf the signal ,fs has been completely eliminated from the present system.

The output of I. F. amplifier S, consisting of signals in the band 6-'1 mc. is applied to a mixer 85, for heterodyning with the output of a frequency swept oscillator St, which includes a frequency modulator of the reactance tube type, the latter being controlled in response to periodic sweep voltages provided by a sweep voltage generator 31. The oscillator 86 may be caused, for example, to sweep the band 16-1'7 mc. periodical- 1y, whence by heterodyne action with the output of I. F. amplier 8d, a 10 mc. I. F. signal is provided which may be amplified in the narrow band amplifier 83, such signal being provided whenever in the excursion of the oscillator 8 over the band 16-17 mc. a signal is encountered in the output of amplifier Sli which differs from the then instantaneous frequency of oscillator 0% by precisely 10 mc. The output of I. F.a1npliiier S8 is applied to a detector 89, which provides signal output signals to an amplier et, the latter signal being applied to the marker electrode 9i of a facsimile type recorder FAX, to provide markings on a record receiving surface 92. The helical platen 93 of the recorder FAX is driven continuously by means of a motor Se, causing the potential marking position of therecorder periodically to scan laterally of the surface 92, a mark being made in response to each signal applied to the marker electrode by virtue of passage of current from the electrode 92to the helical platen 93, via the record receiving surface 92.

The shaft 95 on which is mounted-the platen 93 is secured a cam 05 which momentarily `closes a circuit marker 91 once-for each revolutionof the shaft, and at a time such that the-intersection of marker electrode Eil and helical element 93 intersect at a point laterally of the record rece'iving surface 92 which corresponds withya zero calibration line 98 extend longitudinally of the surface 92 V adjacent to and parallel to Vone edge thereof'.' .Ici :t

Closure of" circuit m'alterillprovides a synchroniaing signalfor theA sweep frequency gentidnnos'cillator, softhat the voltage generated 11' thereby, and therefore the frequency variations of the frequency modulated oscillator 86, may be synchronized and coordinated with the scanning action of the helical platen of the recorder FAX.

Additionally to the recorder FAX is provided a cathode ray tube indicator |00, having vertical deection plates and horizontal deflection plates |02, the plates |0| being connected to the output of the amplifier 00 and the plates |02 being connected to receive output voltage from the generator 81.

It will be clear, with the arrangement as described, that the band of altitude representative signals in the spectrum 6-'1 mc. will be explored periodically for the presence of altitude representative signals, in a manner common and well known in the art of panoramic reception, and that for each frequency in the spectrum will correspond a lateral position of the beam of a cathode ray tube indicator |00 and a recording position of the facsimile recorder FAX laterally of the record receiving surface 92. Discovery of an altitude representative signal results in impression of a voltage on the vertical plate |0| of the cathode ray indicator |00 and on the marker electrode 0| of the recorder, resulting in a vertical deiiection of the cathode ray indication at a lateral position corresponding with the received altitude representative lfrequency, and resulting at the recorder in a permanent mark at a position laterally of the record receiving surface Which corresponds with the received altitude representative frequency. The record receiving surface 92 is time fed, so that, in the course of the operation of the system a series of continuous recordings is provided each representing a time record of the altitude of an aircraft.

Cod-ed interruptions of transmissions from the various aircraft, as provided by coder I2 (Figure L l), result in coded interruptions in the time records, from which the individual records may be identified as originating from specific aircraft, and the record receiving surface 92 may be precalibrated in terms of altitude, as may be also the face of the oscilloscope |00, to provide a refference With respect to which indications may be interpreted.

Since the measurements of altitude as provided by the aneroid cells aboard the various aircraft provide, in actuality, measurements of local atmospheric pressure and since they are precalibrated for standard conditions of pressure, it is necessary to effect compensation at the ground indicator and recorder for variations of local atmospheric pressure from standard. This is accomplished by varying the minimum or zero altitude frequency of the oscillator 86 by means of a trimmer condenser |02, which serves to vary slightly the tuning of oscillator 86, in response to mechanical control by an aneroid cell |03. Thereby, the minimum frequency of oscillator 80, which under standard conditions of atmospheric pressure may be 16 mc. may be varied in direct proportion to changes in atmospheric pressure to precisely the same extent as 4are the airborne aneroid cells i3, and complete compensation of indications is effected by virtue of the similar variations of tuning in response to variations of atmospheric pressure, which occur at the ground and aboard the various aircraft of the system.

The signal provided by the I. F. amplifier 84 may be amplified further by :an I. F. stage |20 which supplies signals to a mixer |2| where it is heterodyned with a further signal supplied by a local oscillator |22, the output of the mixer |2| being applied to a narrow band or voice channel I. F. amplifier |23. The oscillator |22 may be manually tunable over a range sufficient lto enable selection for any signal occuring in the band 6-'1 mc., and representative of the altitude of an aircraft, for transfer to the amplifier |23, and by arranging the response of the amplifier |23 to be suiiiciently narrow, resolution may be accomplished between signals representative of -aircraf t at closely adjacent altitudes. So, for example, the oscillator |22 may be tunable over the band of frequencies 5-6 mc. inclusive, and amplifier |23 may be tuned sharply to a frequency of 1 mc. If then, the pass band of amplifier |23 is 1,000 C. P. S. wide, or .25% of the total pass band, signals may be resolved from aircraft which are separated from other aircraft by a factor of .1 of the total altitude which is monitored in the present system. If that total altitude be taken to be 10,000 ft. resolution to within about ft. may be effected. For a pass-band of 5000 cycles in amplifier |23, resolution to 50 ft. may be effected, but such a narrow pass band would, of course, exclude the frequency Fsc, as presently Selected.

The signal selected by tuning of oscillator |22 may, after amplification in I. F. amplifier |23, be detected in a detector |24, 1amplified in audio amplifier |25 and applied to phones or the like |26 for aural presentation.

The output of the I. F. amplifier |23 may be applied, further, to a discriminator |21, tuned to the mid frequency of the I. amplifier |23, and which provides control voltage to a frequency control element |28, which controls the oscillator |21, maintaining its frequency such that any signal, once tuned in by hand to provide a response in I. F. amplifier |23, is maintained tuned in by the automatic frequency control action of the discriminator |21 and the frequency control element |28. The specific character of the A. F. C. system utilized in the present invention is not of importance, and any system known in the prior ar-t may be utilized, including those which operate wholly electronically, and those in which control is effected mechanically by means of a reversible tuning motor controlled by means of a polarized relay, or the like. Specific examples of systems which I may utilize are disclosed in U. S. patent to M. Wallace, No. 1,878,737.

Suffice it to state, for the purposes of the present application for U. S. patent that the elements |2|, |22, |23, |21, |28 act precisely as a conventional superheterodyne receiver which is subject to automatic frequency control, to maintain itself tuned to a selected signal despite frequency Variations of that signal.

If, now, we assume that the elements |2|, |22, |23, |21, |28 maintain conversion of any selected signal deriving from the I. F. amplifier |23, it will be clear that once the oscillator |22 has been manually tuned to accomplish reception of signals from an aircraft at a given altitude, reception of signals from that aircraft will continue despite changes in that altitude, and that signals from -aircraft flying at altitudes which are slightly different from the selected 'altitude will not be received.

It will be recalled that the signal received by the antenna 80, and which after being converted in frequency is present in the output of I. F. amplifier |23, contains a modulation Fsc at 10,000 cycles per second, Fc, and also by a 60 cycle per second amplitude modulation, Fp, which has a '13 phase which is a function of theazimuthal bearing of antenna 80 with respect to an omni-directional beacon transmitter (Figure 2). This signal is applied over a lead -I 29, to a pair of band-pass filters |30 and I3I, in parallel, the lter I3ll passing the 10,000 cycle per second vsignal Fsc and being sufficiently narrow just barely to pass modulations of that signal. The band-pass lter I3 I, on the other hand, passes only 30 C. P. S. signal and excludes any voice modulation components otherwise present in I. F. amplifier |23. Use of filters |30 and ISI therefore assures that the minimum of extraneous effects will be present in the sign-als required for measurement of range and bearing, and while perhaps superfluous theoretically, actually are of considerable value in improving the laccuracy and steadiness of indications provided by the present system.

At the output of the filters |30 and I3| are connected in parallel a plurality of measuring devices |40 and |4| and meter type indicators |43, |44, one of the latter being connectedfto the output of each of the measuring devices.

The device |4| and its associated meter |44 are precisely similar to the respectively corresponding elements 9 and I0 (Figurel) and will be described in detail hereinafter in conjunction with Figure 4 of the drawings. Briefly described the elements |4I and 9 (Figure 1) include circuit elements for abstracting from vthe frequency modulated signal Fsc, its 60 cycle per second modulation Fc, for comparing the phase of that modulation with the phase of an azimuth corresponding 60 cycle per second'signal Fp, and for indicating the value of the phase relation by means of meter |44 against a scale calibrated in terms of angular bearing.

The device |40 is a phase comparison device which includes a pair of extremely narrow-band filters tuned to 10,000 cycles and capable of eX- cluding modulation of the 10,000 C. P. S. signal Fsc, i. e. having an effective pass-band of not more than 100 cycles. The signal Fsc is supplied to the phase comparison device |40 both from the I. F. amplifier |23, over lead I2s, :and from the oscillator 54 (Figure 1) over lead |45, and the result of the phase comparison is indicated on the meter |43, which is calibrated in range, the scale reading from to 10 miles.

Referring now to the drawings, and particularly to Figure 4 thereof, there is illustrated a bearing analysis system which corresponds with element I 4I of Figure 3. Input signals forthe device are provided over a lead |29, which provides signal deriving from the output of I..F. amplifier |23 (Figure 3) and including la 10 kc. sub-carrier Fsc frequency modulated yat 60 cycles Fc, lthe phase of which is'omni-directi'onally constarrt, and also the signal Fp, which is a 60 cycle per second signal havingv a phase dependingon lthe azimuthal bearing with respect to antenna system 52, 59, IBI] (Figure 2) 'at which is located receiver R. (Figurel.)

TheV signals -Fsc andv Fp are separatedby a pair of filters |50 and I5|',connected inV parallel to the lead |29, the filterv |50v being sulficiently broad to pass signal Fsc with its modulation and the filter comprising an amplifier tuned to the frequency Fp. A

VThe output of the filter |50 s-applied tol a discriminator I 52, -tunedto the frequency Fsc, andwhichV abstracts therefrom the modulation signal Fc,applying that signal overlanamplier |53, Atuned-to thejfrequency Fc,"to a phase comparator |54. The output of amplifier-|5|^-is likewise applied to the phasecomparator |54, andthe outputlof the comparator then has a magnitude and a sense dependent on the phase relation of the signals Fc and Fp. The output of the comparator |54 may be measured on a meter |44, which may be of the type which provides right and left readings with respect to center in accordance with the magnitude and sense of the voltage or current applied thereto.

The bearing analyzing system |4I is in no sense my invention, but is per se well known, and has been thoroughly described in the literature pertaining to air navigation, as part of the standard CAA approved omni-directional beacon system.

In Figure 5 is illustrated a phase comparison system which has for its ultimate purpose the provision of range information. For this purpose, as has been hereinbefore fully explained, it suffices to compare the phase of the ysignal Fsc as transmittedY with the phase of that same signal as received' afterY being transponded.

There is accordingly supplied to the system |40 a signal Fscs deriving from generator 54 (Fig. 2), and applied over a lead |45, which represents -the signal yas transmitted, and a further signal Fscpf'which is applied over lead |29, which is applied over lead |29 and is derived from the I. F. amplifier |23. The signal Fscs may be considered to be of standard or reference phase, whereas the phase of the signal Fscp is a function of distance or range.

A difficulty, which has been beforealluded to, arises here, in that the signals Fscs and Fscp are frequency modulated and' hence have no constant phase which may be measured. This difculty is eliminatedby eliminating from the signals Fscs and Fscp the frequency modulation inherent thereon, and this may be simply accomplished by passing the signals through lters |60of veryv narrow pass-band, i. e. less than v:Li-'6.0 cycles.4 Since the frequency modulated signals Fscs andiisp may each be considered to be made up of a carrier of unvarying frequency, plus a series of sidebandsA spaced from the carrier by increments of 60 cycles, the modulation frequency of the carriers, this step provides signal of pure unvarying sine character, which are readily adaptable to phase comparison.

The outputs of the filters |60 may be applied to a suitable phase comparator IGI, of the same nature as comparator ,|54 (Figure 4) and the output signal indicated on a meter |43, similar to meter |44, but which may vbe suitablycalibrated in miles, or thousands of yards, or the like.

. The individual channel for range, bearing and voice which I havey denoted. I. C. is of course adequate. forv providing information only with respect to )awsingle aircraft. The present systemmay be required to. provide .similar information concerning a considerableY number of aircraft,A and tovacco'rnplivsh thisJfunctionV I provide apluralsity 0f .QahQFe OlQWs'SJW @ther SO" lating amplifier5,coupled-in4 parallel tothe outeach isolating amplifier'4 Ililumay be coupled a completechannel'I; C.- similartothat illustrated, and each being tunablexselectively to enable reception on a given altitude frequency,;and hence from a selected aircraft.

In order to assure that any specific I. C. channel is intune'with af selected aircraft, or in tunefftola selected valtitude -representative .fre-

quency, the output of the oscillator |22, which is variable over the frequency band -6 mc., and tuning of which serves to select the frequency of the associated I. C. channel, is applied to a mixer |80, to which is also applied the output of a fixed frequency oscillator |8I, which supplies a frequency of exactly 1 mc. The output of the mixer |80 then comprises a single fixed frequency selectable in the range 6-7 mc. by tuning the oscillator |22. The output of the mixer |80 may be applied over a circuit maker and breaker |82 to the input of mixer 85, where it is treated exactly as an altitude representative frequency and is applied to the recorder FAX and to the indicator |00, for comparison with actual altitude representative signals.

If then, we assume that an aircraft at altitude 6000 ft. is to be selected for observation and placed in communication with a selected channel I. C` the switch |82 associated with that channel is closed and the oscillator |80 manually tuned until the pip produced thereby on the indicator |00, and the mark produced thereby on the record receiving surface 92 coincide with the pip and the mark due to the aircraft at 6000 ft. The switch |82 may then be opened, and manual control of the oscillator |22 surrendered to the automatic frequency control elements |21 and |28.

While the above description has been confined to a system of range determination which utilizes the carrier of the signal fsc, at 10,000 cycles per second, for range determination, this obviously is not essential to the present system. In fact, for many purposes it is far from an ideal choice, having primarily the merit that the signay isc is inherently available in the C. A. A. omni-directional beacon system, as at present formulated. Utilization of a lower frequency than kc. for range determination enables an increased range to be measured without ambiguity. For example, if a frequency of 5 kc. is utilized, range may be measured to about miles, and if a frequency of 3 kc. is utilized, to more than miles.

Further, the use of a 10 kc. frequency modulated signal requires the use of filters of high Q, to remove from the 10 kc. carrier its sidebands due to modulation. The use of such lters introduces the possibility of phasing errors due to frequency drift of the filters.

I therefore conceive it to be desirable, ii not preferable, to introduce into the beacon transmitter of Figure 2, a source of 3 kc. signal which may be imposed on the 120 mc. carrier as an amplitude modulation, and which may be transponded by the airborne transponders of Figure l, and detected and compared in phase with a signal of standard phase for range determination, at the ground station, in accordance with the disclosure and techniques described hereinbefore.

The use of such a signal requires that no speech frequencies at or adjacent the 3,000 cycle signal be permitted anywhere in the system, which may readily be accomplished by means of suitable filters. So filter 2| (Figure 1) must cut off at 2500 cycles instead of at 4000 cycles, and detector 'I (Figure 1) must be provided with a single frequency rejection filter tuned to 3 kc. to delete 3 kc. tone from reproducer 8.

In Figure 2 the microphone 6| may cut off at 2500 cycles.

In Figure 3 the amplifier |25 may cut off at 2500 cycles, and the filter |30 must be tuned to 3 kc., and in Figure 5 the filters |60 will be un- 16 necessary and may be omitted, but if used must be tuned to 3 kc., and may be relatively broad band, to avoid the possibility that ,phase shifts will be introduced thereby.

I consider the above described modifications to be of relatively obvious character, and that the system utilizing the 10 kc. FM. signal and utilizing the 3 kc. unmodulated signal for range determination, are broadly or generically similar.

While I have described an embodiment of my invention and have suggested certain modifications thereof, it will be clear that further modifications and variations of the system as a whole, as well as of various of its details, may be resorted to without departing from the spirit and true scope of the invention, being such as will readily suggest themselves t-o those skilled in the pertinent art after my invention and system has been disclosed.

What I claim and desire to secure by Letters Patent of the United States is:

1. In combination, a Variable frequency oscillator, means for controlling the frequency of said oscillator in accordance with the value 0f a telemetric quantity, a source of master frequency, means for combining the frequency of said source and of said oscillator to provide a further frequency representative of the value of said telemetric quantity, means for detecting the vvalue of said telemetric quantity and including a superheterodyne receiver, and means for supplying said master frequency to said superheterodyne receiver for heterodyning with received signals, whereby said means for detecting is independent of variations of said master frequency.

2. In combination, a plurality of independent variable frequency oscillators, a 4source of stable signal frequency, means for separately combining said signal frequency and signals derived from each of said oscillators to .provide a plurality of variable frequency signals, means for transmitting said variable frequency signals, means for controlling the frequency of each of said plurality of independent variable frequency oscillators in accordance with the value of a telemetric quantity, and means for receiving and analyzing said variable frequency signals to determine values of said telemetric quantities, said last means comprising a superheterodyne receiver, and means for supplying said master frequency to said superheterodyne receiver for heterodyning with said variable frequency signals, whereby drifts of frequency of said source of signal frequency affects equally the determination of al1 values of said telemetric quantity.

3. In combination, a beacon transmitter providing an alternating current signal having a phase of different value for each azimuthal bearing angle from said transmitter, means located remotely of said beacon transmitter for repeating signals derived from said beacon transmitter, the value of said phase being retained in repeating, and means remote from said means for repeating for receiving said repeated signals and for deriving therefrom information concerning the azimuthal bearing angle of said means` for repeating.

4. The combination in accordance with claim 3 wherein said beacon transmitter provides an omni-directional transmission at a fixed carrier frequency and having lone modulation at xed phase omni-directionally and another modulation having a phase which varies in accordance with the direction from which the transmitter is viewed.

5. The combination in accordance with claim 3 wherein said means for repeating comprises a heterodyning circuit for translating the repeated frequency, and wherein means are provided for controlling the said heterodyning circuit for providing repeated signals at a frequency determined in accordance with a value of a measurable quantity.

6. In combination, means for transmitting a radio frequency signal having a modulation impressed thereon, means for repeating said signal including said modulation from a location'remote from said means for transmitting to provide a repeated signal, said means for repeating including means for translating the frequency of said repeated signal to a new frequency having a numerical value representative of the value of a measurable quantity, means remote from said means for receiving said repeated signal and said radio frequency signal and for deriving from said repeated signal and said radio frequency signal the value of said measurable quantity, said means for receiving including means for comparing said modulation as transmitted by said means for transmitting with said modulation as received by said means for receiving to derive a measurement -of range of said means for repeating.

7. In combination, an omni-directional beacon for transmitting an omni-directional carrier modulated with a sub-carrier which is in turn modulated with a first fixed frequency and for transmitting a directional beam of radiant energy rotating at the said xed frequency a repeater located remotely of said beacon for receiving signals derived therefrom and for re-transmitting a further carrier modulated with said modulated sub-carrier and with a modulation derived from rotation of said directional beam of radi-ant energy, means for adjusting the frequency of said re-transmitted carrier to a frequency value corresponding With the value of a telemetric quantity, and means for receiving said re-transmitted carrier and for interpreting the carrier frequency of said first mentioned carrier and the modulations of said re-transmitted carrier as received by said means for receiving in terms of a value of said telemetric quantity, a range, and a bearing.

8. In combination, an omnidirectional range radio beacon transmitter for transmitting a carrier and a rst modulation of said carrier yand a second modulation of said carrier, airborne means for receiving said carrier and for converting said carrier to a further carrier having a frequency determined in accordance with the altitude of said airborne means and for re-transmitting said further carrier modulated by said first and second modulations, a receiver located remotely of said Iairborne means for receiving said further carrier, and means responsive to said receiver for analyzing said further carrier and its modulations in terms of a range, an altitude and a bearing of said airborne means.

9. In combination, ground located means for transmitting signal at a first carrier frequency, means located aboard an aircraft for receiving said signals, means aboard said aircraft for measuring the altitude of said aircraft, means responsive to said last named means and to reception of signals by said means for receiving for transmitting from said aircraft a further signal having a frequency corresponding with altitude of said aircraft, a ground receiver for said further Signal, and means for controlling said ground receiver to receive only said further signals having carrier frequencies falling within a predetermined relatively narrow band of frequencies corresponding With a relatively narrow range of altitudes, said receiver comprising means for heterodyning the frequency of said further signals with the frequency of said first carrier frequency.

110. In combination, ground located means for transmitting signal at a first carrier frequency, means located aboard an aircraft for receiving said signals, means aboard said aircraft and responsive to said signals for transmitting a further signal having a further carrier frequency representative of altitude of said aircraft, a predetermined range of further carrier frequencies represented a predetermined range of altitudes, a ground receiver for said further carrier frequencies, said ground receiver having means for receiving selectively only predetermined ones of said further carrier frequencies representative of a predetermined portion of said range of altitudes, said ground receiver comprising means for heterodyning the frequency of said first carrier with the frequency of said further carrier.

11. In combination, a variable frequency oscillator mounted on an aircraft, means for controlling the frequency of said oscillator in accordance with the altitude of said aircraft, a source of master frequency, means aboard said aircraft for receivingsaid master frequency and for combining the frequency of said source and said master frequency to provide a further frequency representative of the altitude of said aircraft, remote means for receiving and detecting said further frequency and comprising heterodyne means, and means for supplying said master frequency to said heterodyne means for heterodyning With said further frequency.

12. In combination, a variable frequency oscillator mounted on an aircraft, means for controlling the frequency of said oscillator in accordance with the altitude of said aircraft, a source of master frequency, means aboard said aircraft for receiving said master frequency and for combining the frequency of said source and said master frequency to provide a further frequency representative of the altitude of said aircraft, remote means for receiving and detecting said further frequency and comprising heterodyne means, and means for supplying said master frequency to said heterodyne receiver for heterodyning with said further frequency, said remote means for receiving comprising means for receiving only selected values of said further frequency.

13. In combination, a plurality of independent variable frequency oscillators, each aboard a different aircraft, a single source of signal frequency, means aboard each of said aircraft for receiving said signal frequency and for combining said variable frequency with said signal frequency to provide a heterodyne frequency, means for ltransmitting said heterodyne frequenciesl means aboard each of said aircraft for controlling the frequency of its variable frequency oscillator in accordance with its altitude, and means for selectively receiving said heterodyne frequencies to derive altitude information therefrom, said means for selectively receiving comprising a superheterodyne receiver, said superheterodyne receiver comprising means for heterodyning said heterodyne frequencies with said signal frequency.

14. The combination in accordance with claim i9 I wherein said source of master frequency is an omni-directional range radio transmitter.

15. In combination, an omni-directional beacon for transmitting an omni-directional carrier modulated by a rst alternating current signal which is of equal phase omni-directionally and a second alternating current signal which has a phase at each bearing from said transmitter which is representative of said each bearing, a repeater located remotely of said beacon for receiving said omni-directional carrier and for retransmitting a carrier modulated with said rst and second alternating current signals, means for receiving said 1re-transmitted carrier and for deriving said first and second alternating eurrent signals therefrom, and means responsive to said first and second alternating current signals for providing an indication of bearing and range of said repeater from said transmitter.

16. The combination in accorda-nce with claim Y 7 wherein said telemetrie quantity is altitude of said repeater.

MARCEL WALLACE.

`20 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Busignies Dec.- 25, 

